From what I can remember when doing my own car, the thicker the wire the better. As far as what gauge, I can't remember but for some reason 4-8 gauge is coming to mind. But go with the lower guage for better conveyance. What RMS rating is your amp? Is it 1 ohm stable? Because that's what matters instead of the Peak wattage.

Yes, it does matter. If the power wire (from the battery) is too thin (lower guage number means thicker wire.. 4 is thicker than 8) then it can cause a fire hazard. Wouldn't go cheap and risk burning your car to a crisp! For 1000W I would say probably 4 guage power wire and make sure you use a "in-line fuse". The speaker wire you can use 16 guage and it should be fine. It also depends on the quality of your equipment, if in doubt stop by cartoys and BB or a car audio shop for a few different opinions.

the12volt.com has a calculator for this sort of thing. I'd say 4 gauge if it's a real 1000watt RMS amp and it's in your trunk. If it's that "max/peak/dynamic" not RMS power rating you could go with 8 gauge especially if it's not that long, like under your seat. FUSE IT AT THE BATTERY and if you're putting in a sub box BOLT or SCREW IT DOWN. Too many people don't and in an accident in an SUV, wagon or van, that box will kill you if you roll over and it gets loose.